Recently, liquid crystal displays, plasma displays and organic EL displays to be used for personal computers, word processors, watches and desk-top calculators tend to be used in sever conditions. Therefore, high durability is required to an optical film such as a polarizing plate protective film, a retardation film, a front filter for plasma display panel and a front film for organic EL display so that the properties thereof is not deteriorated so that, for example, the physical properties are not degraded and the dimensional stability is maintained, in the sever environment.
Besides, a problem of narrow viewing angle has been posed hitherto about the liquid crystal display. Therefore, use of various kinds of retardation film has been proposed for expanding the viewing angle; cf. Patent Documents 1 to 5, for example. It is well known that a cellulose ester having a lower substitution degree exhibits a larger Rt (retardation in the thickness direction of a film) value (refer to Patent Document 6), however, in an in-plane switching mode liquid crystal display, desired is a cellulose ester film having a lower Rt value, in order to enlarge a viewing angle.
The retardation film strongly influences on the displaying properties such as the viewing angle, color tone and the gradation of the display. Accordingly, high dimensional stability during a long time use and high stability in the retardation are desired to the retardation film. Moreover, high dimensional stability of the polarizing plate using the retardation film, particularly the dimensional stability in the absorption axis direction of the polarizers and superior resistivity against degradation of the polarizing plate, are desired. The cellulose ester film disclosed in Patent Document 7, being added with a polyester, exhibits a superior dimensional stability, however, durability of polarizing plate is poor. The cellulose ester film disclosed in Patent Document 8, being added with an acryl polymer, exhibits a lowered Rt value, however, notable deterioration of the polarizing plate after a long time use has been observed. In order to prevent deterioration of a polarizing plate, generally known is to add a material containing an aromatic ring, however, addition of an aromatic ring results in a problem in that Rt value is increased.
When the dimensional variation during the aging for a long time is large, stress is generated between the polarizing plate and the adhesive or between the polarizing plate and the liquid crystal cell pasted on the polarizing plate through an adhesive layer, and a phenomenon so-called corner unevenness which is a white defect in a black image is caused (refer to Patent Documents 9 and 10).
It is desirable that the retardation film not only has a function of optical compensation for the liquid crystal cell but also is superior as the protective film with respect to the durability of the flatness against an environmental change.    Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereafter referred to as JP-A) No. 4-229828    Patent Document 2: JP-A No. 4-258923    Patent Document 3: JP-A No. 6-75116    Patent Document 4: JP-A No. 6-174920    Patent Document 5: JP-A No. 6-222213
Patent Document 6: JP-A No. 2002-120244    Patent Document 7: JP-A No. 2002-22956    Patent Document 8: JP-A No. 2003-12859    Patent Document 9: JP-A No. 5-341124    Patent Document 10: JP-A No. 6-167611